1. Field of the Invention
The present invention relates generally to superplastic forming and more specifically to controlling the gas pressure during forming.
2. Description of Related Art
Superplastic forming (SPF) takes advantage of a material's superplasticity or ability to be strained past its rupture point under certain elevated temperature conditions. Superplasticity in metals is defined by very high tensile elongations, ranging from two hundred to several thousand percent. Superplasticity is the ability of certain materials to undergo extreme elongation at the proper temperature and strain rate. SPF is a process used to produce parts that are difficult to form using conventional fabrication techniques.
SPF typically is accomplished by heating a sheet of material to a point of superplasticity, clamping the material within a sealed die and then using inert gas pressure applied to one side of the sheet of material to force the material to stretch and take the shape of the die cavity. Pressure is controlled during the process to ensure the material maintains an appropriate deformation rate for superplasticity at the elevated temperature. Accordingly, superplastic materials can be stretched at higher temperatures by several times their initial length without breaking.
Typically SPF applications while having advantages over conventional stamping techniques, including increased forming strains, reduced spring back and low tooling costs, also have disadvantages in that they are limited to low volumes as they have relatively long cycle times. Specifically, a conventional SPF process used to manufacture a complex part can require a cycle time as high as 30 minutes.
Conventional SPF systems are relatively slow in terms of pressurization and have only moderate control of forming pressure. Early embodiments of SPF used a constant forming pressure. Once pressure in the die reached a target or predetermined target pressure, the pressure was held constant until the workpiece was formed by the gas pressure pressing the workpiece against the forming surface of the cavity. The use of a constant pressure throughout the forming cycle calls for long forming times. While faster forming times might be achieved if the pressure was simply increased during forming, there are periods when the forming process requires a relatively low pressure, typically at those points where the workpiece makes contact with the die surface, or when the material is formed at a rate where it may split or crack.
Prior systems were of the type having the ability to increase the pressure in the die cavity. These systems have a drawback. Once the pressure is increased, there is no way to lower the pressure in an accurate manner other than at the end of the forming cycle when the pressure is typically reduced through the activation of a quick exhaust or dump file. Thus the gas pressure profile could either be constant or increased through the forming cycle but not lowered.
Accordingly, in order to better utilize the SPF process for higher production volumes, such as those used in the automotive industry, it is critical that the process cycle time be reduced.